This disclosure relates to techniques for processing materials in supercritical fluids. Embodiments of the disclosure include techniques for material processing in a capsule disposed within a high-pressure apparatus enclosure. The methods can be applied to growing crystals of GaN, AlN, InN, and their alloys, including, for example, InGaN, AlGaN, and AlInGaN, and others for the manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation devices, photodetectors, integrated circuits, and transistors, among other devices.
Large area, high quality crystals and substrates, for example, nitride crystals and substrates, are needed for a variety of applications, including light emitting diodes, laser diodes, transistors, and photodetectors. In general, there is an economy of scale with device processing, such that the cost per device is reduced as the diameter of the substrate is increased. In addition, large area seed crystals are needed for bulk nitride crystal growth.
There are known methods for fabrication of large area gallium nitride (GaN) crystals with a (0 0 0 1) c-plane orientation. In many cases, hydride vapor phase epitaxy (HVPE) is used to deposit thick layers of gallium nitride on a non-gallium-nitride substrate such as sapphire, followed by the removal of the substrate. These methods have demonstrated capability for producing free-standing c-plane GaN wafers 50-75 millimeters in diameter, and it is expected that GaN wafers with diameters as large as 100 millimeter can be produced. The typical average dislocation density, however, in these crystals, about 106-108 cm−2, is too high for many applications. Techniques have been developed to gather the dislocations into bundles or low-angle grain boundaries, but it is still very difficult to produce dislocation densities below 104 cm−2 in a large area single grain by these methods, and the relatively high concentration of high-dislocation-density bundles or grain boundaries creates difficulties such as performance degradation and/or yield losses for the device manufacturer.
The non-polar planes of gallium nitride, such as {1 0 −1 0} and {1 1 −2 0}, and the semi-polar planes of gallium nitride, such as {1 0 −1 ±1}, {1 0 −1 ±2}, {1 0 −1 ±3}, and {1 1−2 ±2}, {2 0 −2 1} are attractive for a number of applications. Unfortunately, no large area, high quality non-polar or semi-polar GaN wafers are generally available for large scale commercial applications. Other conventional methods for growing very high quality GaN crystals, for example, with a dislocation density less than 104 cm−2 have been proposed. These crystals, however, are typically small, less than 1-5 centimeters in diameter, and are not commercially available.
Legacy techniques have suggested a method for merging elementary GaN seed crystals into a larger compound crystal by a tiling method. Some of the legacy methods use elementary GaN seed crystals grown by hydride vapor phase epitaxy (HVPE) and polishing the edges of the elementary crystals at oblique angles to cause merger in fast-growing directions. Such legacy techniques, however, have limitations. For example, legacy techniques do not specify the accuracy of the crystallographic orientation between the merged elementary seed crystals they provide a method capable of producing highly accurate crystallographic registry between the elementary seed crystals and the observed defects resulting from the merging of the elementary seed crystals.
Conventional techniques are inadequate for at least the reason of failing to meaningfully increase the available size of high-quality nitride crystals while maintaining extremely accurate crystallographic orientation across the crystals.